Normal light, such as natural light (e.g., sunlight) or light emitted from general light-sources (e.g., an electric lamp) is white light, which has an essentially uniform spectral distribution.
On the other hand, optical materials or optical devices such as an optical film, an optical collimator, a polarization-converting film or a liquid crystal display usually use light in a particular wavelength range. Accordingly, the optical materials or optical devices use the normal light with poor efficiency.
A wavelength-selective reflection film has been proposed to improve the efficiency. The wavelength-selective reflection film selectively reflects light in a particular wavelength range, and selectively transmits light in another wavelength range. The optical materials or optical devices can use the reflected (or transmitted) light in the particular wavelength range. The transmitted (or reflected) light in another wavelength range, which has not been used in the optical materials or optical devices, can be used in another materials or devices, or can be converted into usable light.
Liquid crystal molecules in a cholesteric phase (Ch or N*) have been proposed to prepare the wavelength-selective reflection film. Japanese Patent Provisional Publication Nos. 9(1997)-304770, 10(1998)-54909, 11(1999)-44816, and International Patent Nos. WO97/16762, WO00/34808 disclose the wavelength-selective reflection film made of liquid crystal molecules in the cholesteric phase.
The wavelength-selective reflection film shows optical characters attributed to the cholesteric phase of the liquid crystal molecules.
First, the film totally reflects counterclockwise circularly polarized light and transmits clockwise-polarized light where the liquid crystal molecules are counterclockwise helically aligned in the cholesteric phase. The film totally reflects the clockwise circularly polarized light and transmits the counterclockwise-polarized light where the liquid crystal molecules are clockwise aligned.
Second, the film selectively reflects light in a wavelength range, which is independent from an incident angle of light.
Third, the film reflects a clockwise circularly polarized incident light as a clockwise circularly polarized reflected light, and reflects a counterclockwise circularly polarized incident light as a counterclockwise circularly polarized reflected light.